1. Field of the Invention
The present invention relates to a sheet feeding apparatus used in an image forming apparatus to feed sheets one by one from a stack of sheets.
2. Description of the Related Art
In the art of sheet feeding apparatus used in image forming apparatus such as printers and copying machines, it is known to use an inclined separation plane to separate a sheet. In this type of sheet feeding apparatus, a sheet is moved against an inclined separation plane by rotating a sheet feed roller in contact with the top of a stack of sheets so that a sheet on the top is separated from the other sheets.
The advantage of the sheet feeding apparatus of the inclined plane separation type is that it has a simple structure because it does not need complicated separation means such as a separation pad and a reversing roller, or a mechanism for maintaining a sheet at a particular height such as a lifting mechanism or a pressure plate urged by a spring.
Although the sheet feeding apparatus of the inclined plane separation type is capable of separating sheets with a thickness and rigidity within certain ranges, it is difficult for this type of sheet feeding apparatus to handle very thin sheets or sheets with very low rigidity. When sheets are very thin, they often bend, and a separation failure often occurs. Thus, a feeding error often occurs in which a plurality of sheets are fed at a time. One technique to solve the above problem is to dispose the sheet feed roller in the vicinity of the inclined separation plane so that a sheet is sharply curved between the inclined separation plane and the sheet feed roller, thereby enhancing the separation ability and preventing a plurality of sheets from being fed at a time. However, in this technique, when a sheet has very large rigidity as is the case with a very thick sheet, the sheet stops when it comes into contact with the inclined separation plane, because such a sheet is not easily bent.
When sheets curl at their leading end, there is a high possibility that a plurality of sheets will be fed at a time or that other feeding failures will occur.
In view of the above, it is an object of the present invention to provide a sheet feeding apparatus of the inclined plane separation type capable of separating and feeding sheets in a highly reliable fashion regardless of the thickness and rigidity of sheets and regardless of whether the sheets curl at their leading end.
According to an aspect of the present invention, there is provided a sheet feeding apparatus comprising sheet supporting means for supporting a stack of sheets; a sheet feed roller, in contact with the upper surface of a sheet at the top of the stack of sheets supported by the sheet supporting means, for feeding the sheet; an inclined separation plane for separating the sheet fed by the sheet feed roller from the other sheets; a rotatable separation roller, disposed between the inclined separation plane and the sheet feed roller, for pressing the upper surface of the sheet fed by the sheet feed roller; and roller supporting means for movably supporting the separation roller.
In accordance with yet another aspect of the present invention, there is provided a sheet feeding apparatus as described above wherein the rotatable separation roller is movable in accordance with the rigidity of the sheet being fed.
In accordance with yet another aspect of Applicants""invention there is provided a sheet feeding apparatus comprising sheet supporting means for supporting a stack of sheets, sheet feeding means for feeding the sheets, an incline separation plane and a rotatable separation roller, all as described above. In accordance with this aspect of the invention, there is further provided switching means for switching the separation roller between a position in which the separation roller presses the upper surface of the sheet being fed and a position in which the separation roller is apart form the upper surface of the sheet being fed, wherein the switching means automatically switches the position of the separation roller depending on the rigidity of the sheet which is fed by the sheet feeding means and bent by the inclined separation plane.
In accordance with yet another aspect of the present invention there is provided an image forming apparatus which includes a sheet feeding apparatus as described above in combination with image forming means for forming an image on the sheet separated by the inclined separation plane.